The control system of this kind is adapted not only to control the internal combustion engine and the electric motor to output power, or to use the electric motor to output the power instead of the internal combustion engine, but also to control a rotational speed of the internal combustion engine using the electric motor thereby driving the internal combustion engine at an optimum fuel economy. For example, the power outputted from the internal combustion engine is distributed to the electric motor side and an output shaft side by a differential mechanism, and a torque of the output shaft side is increased by a reaction resulting from using the electric motor as a generator. The generated electric power is utilized to drive a vehicle. According to the hybrid drive unit thus structured, the power outputted from the internal combustion engine has to be converted into the electric power. Therefore, if an amount of such power conversion is increased, a power loss resulting from the power conversion is thereby increased. In order to avoid such disadvantage, according to the prior art, a power transmission mode of the drive unit, that is, a driving mode (or running mode) is shifted according a driving condition.
For example, Japanese Patent Laid-Open No. 2005-125876 discloses a hybrid vehicle driving device. Specifically, according to the teachings of Japanese Patent Laid-Open No. 2005-125876, the hybrid vehicle driving device comprises an input element to which an internal combustion engine is connected, a reaction force element to which a first motor/generator is connected, a power distribution mechanism for performing a differential action having an output element connected to a second motor/generator, and two clutches for connecting an output member selectively with the output element and the reaction force element. Therefore, according to the hybrid vehicle driving device taught by Japanese Patent Laid-Open No. 2005-125876, the engine torque is outputted to the output member while being amplified by the power distribution mechanism, and the driving mode is shifted between a driving mode in which a speed change ratio is varied continuously and a driving mode in which a vehicle is driven by using any of the motor/generators as a prime mover.
According to the teachings of Japanese Patent Laid-Open No. 2005-125876, the driving mode (or running mode) is changed according to an engagement status of the two clutches. Therefore, the rotational speeds and power transmission efficiencies of the engine and the motor/generators can be optimized by engaging or disengaging the clutches in accordance with a driving condition of the vehicle such as an opening degree of an accelerator or a speed of the vehicle. However, the vehicle may be kept driven by the power of the engine during a mode change. In addition, in case of arranging a speed change mechanism for the purpose of diversifying the driving modes, the vehicle may also be kept driven by the power of the engine during a mode change executed by carrying out a speed change of the speed change mechanism. In those cases, an operating condition of the engine, that is, as an operating point of the engine is governed by a speed of the vehicle, drive demand etc. This means that the operating condition of the engine may become unacceptable condition. For example, the rotational speed of the engine may be lowered to the speed lower than a predetermined allowable speed, otherwise, the operating condition of the engine may enter into an operating range where noise or vibration exceeds an allowable level. In such cases, the operational point of the engine has to be changed compulsory if the mode is not changed. As a result, fuel consumption may be degraded, or acceleration and deceleration caused by such change in the operational point of the engine bring an uncomfortable feeling.